Corals are ocean-dwelling invertebrates in the same phylum as jellyfish. Corals are tiny and create an exoskeleton that is fixed to something hard, like the remains of previously existing corals. So these organisms build up a geological stratum, a reef, beneath the surface of the sea, often close enough that parts of the reef are exposed at the lowest water level. The coral reef system is the substrate for one of the Earth’s major ecological zones.

Corals are symbiotic with a single celled dinoflagellate, a kind of algae that combines available nutrients such as ammonia and the photosynthetic process using sunlight to grow, maintain, and reproduce. These algae provide the coral with nutrients, and the waste products produced by the corals are the nutrients used by the dinoflagellates. Depending on the species, corals may also trap tiny organisms and eat them. There are many species of both symbionts, there are multiple possible combinations of symbionts that work, and it is all very complex.

Under certain conditions, the corals are unable to provide the symbiont algae with nutrients, so the latter either die or simply abandon the relationship. Reduction in nutrients provided by the dinoflagellates further reduces the coral’s bioactivity, worsening the situation, in a kind of downward spiral.

The algae symbionts provide the coral structure with its famous color, so when they abandon the relationship, the primary color of the coral structure is white, so the process is referred to as bleaching.

There are a lot of things that can cause bleaching, including disease, physical damage by storms, changes in water chemistry, and warming of the waters. Sea temperatures are elevated because of surface warming caused by human released greenhouse gas pollution, so corals around the tropics are generally more susceptible to bleaching than they have been in known history, and many mass bleaching events have been observed over the last 20 years or so. During El Niño years, ocean temperatures in certain regions can go even higher, so El Niño years are typically associated with numerous mass bleaching events. This year, we have extremely elevated ocean temperatures caused by anthropogenic global warming, in combination with an exceptionally strong El Niño, and this has caused the Great Barrier Reef, the world’s largest reef ecosystem, to crash.

According to the University of Queensland Global Change Institute Director Ove Hoeghguldberg, “From the tip of Cape York to the Whitsundays, the Great Barrier Reef in the east to the Kimberley’s in the west and Sydney Harbour in the south, Australia’s corals are bleaching like never before. This is the worst coral bleaching episode in Australia’s history, with reports of coral dying in places that we thought would be protected from rising temperatures.”

Coral scientist Tyrone Ridgeway adds, “Previously, scientists thought the reefs off Western Australian could withstand bleaching and that southern waters around Sydney would be too cool for bleaching — this year has shown that is not the case. It will already take several decades for coral reefs to recover from this bleaching event.”

As noted, coral reefs form the basis for a major ecosystem, but also, for a major economy. There are lots of places in the world that people visit almost entirely for the corals, or some natural feature related to the corals. In Australia, some 69,000 people are employed in a five billion dollar industry of coral ecotourism and aquaculture.

Reefs can recover. Somewhere out there on the reef there are a few surviving corals, and a few surviving dinoflagellates. If conditions return to normal, they may start to recolonize the reef surface. However, it is also possible that the coral ecosystem can be replaced with an algae mat ecosystem across large areas. Living coral reefs maintain their relative position in relation to the sea surface, and thus provide barrier effects and control the geomorphology of a huge ecosystem. An algae mat ecosystem would presumably erode more than grow (except in very protected areas), and between that and sea level rise, the barrier effect would be significantly reduced.

Also, it takes months to years for recovery to occur, and the worse the bleaching the longer it takes. We are probably entering an era where heat stress bleaching will become much more common, and more severe when it does occur. At the same time, severe and physically large tropical storms are becoming much more common in the Pacific, so the chance of a second hit from this effect during recovery is increased. In other words, over the next few decades, a major reef like the Great Barrier Reef may become bleached more often than not over much of its area. Eventually, as sea temperatures continue to warm, it may simply become impossible to maintain such reefs.

There is some hope in that dinoflagellates that can withstand warmer conditions could become predominant, or even evolve. Perhaps in a few centuries from now, reefs will adjust to new conditions. On the other hand, climate change results in higher variability of temperature conditions, not just an increase, which would make such an adaptation difficult. Keep in mind that during recent history of life (over a few hundred million years or so) there have always been reefs, but not always made and maintained by corals. The organisms that produce this important ecosystem have, in the past, gone extinct and been replaced by entirely different systems, several times. That replacement was unlikely to have been a neat and efficient process.

More information here.